Introduction

Heavy metal pollution caused by various industries (mining, metallurgy, electroplating, high technology industries, and others) is a serious threat to ecological systems and to the health of its inhabitants. Meanwhile, increasing demand for metal resources results in the scarcity of metal resources in the natural environment. Thus, researchers are addressing the growing need for recovering heavy metals from wastewater to remove the toxic chemical elements and to recycle these metals (Wang and Chen 2009; Zhang et al. 2010). Conventional methods, such as chemical precipitation, ion exchange, electrochemical treatment, and active carbon adsorption, have been tried and improved. However, problems such as low efficiency, high costs, complex processes, and secondary pollution have not been solved thoroughly. In recent years, increasing attention has been paid to biosorbents because of their excellent adsorption properties, low cost, and high availability (Das 2010; Selenska-Pobell and Merroun 2010; Wang and Chen 2009; Xiao et al. 2012, 2010). Nevertheless, separation and recovery of biosorbents from solutions are still challenges in the field (Das 2010). The case of magnetotactic bacteria (MTB) as promising biosorbents was investigated because MTB can be easily separated from wastewater using magnetic fields (Song et al. 2008, 2007; Wang et al. 2011; Xie et al. 2009).

MTB refer to a heterogeneous group of prokaryotes that can orient and migrate along geomagnetic field lines (Schüler and Frankel 1999). They are capable of forming a specific membrane-enveloped intracellular structure called magnetosome (Frankel 2003; Tanaka et al. 2010). Magnetosomes have uniform, species-specific crystal habits with narrow size and shape distributions, and they are generally aligned in chains within bacteria (Tanaka et al. 2010). Magnetite (Fe3O4) or greigite (Fe3S4) are magnetic particles that comprise magnetosomes (Bazylinski et al. 1995; Mann et al. 1990; Schüler and Frankel 1999). MTB widely exist in freshwater, sediments, and oceans, among others (Lin and Pan 2009; Mann et al. 1990; Matsunaga et al. 2005; Sakaguchi et al. 2002). Most of them are microaerophilic and are found in or below the oxic–anoxic transition zone (OATZ) (Flies et al. 2005; Lefevre et al. 2009; Li et al. 2007). Compared with studies on the applications of MTB in the medical and environmental fields, few papers have focused on MTB application in environmental remediation (Bazylinski and Schubbe 2007; Kopp and Kirschvink 2008; Linford et al. 2005; Peng et al. 2000; Xie et al. 2009). Studies focusing on removing metals and trace radionuclides from wastewater using MTB were first presented by Bahaj et al. (1994, 1998a, 1991, 1998b). Subsequently, the adsorption of heavy metals was further investigated (Keim and Farina 2005; Song et al. 2008, 2007; Wang et al. 2011). The mineralization of metal ions occurs in MTB after exposure to metallic elements other than iron (Arakaki et al. 2002; Cai et al. 2011; Song et al. 2008; Tanaka et al. 2010). Moreover, MTB showed selective reinforced competitive biosorption for Au (III) and Ag (I) in Au–Cu and Cu–Ag binary ion systems (Song et al. 2007; Wang et al. 2011).

Confined by the harsh growth conditions of MTB, only a few strains are available in pure culture. Magnetotaxis can be lost in continuous culture or after prolonged storage at 4 °C (Heyen and Schüler 2003; Li et al. 2010, 2007; Lin and Pan 2009; Sun et al. 2008). However, Liu et al. (2010) achieved large-scale production of magnetosomes of Magnetospirillum gryphiswaldense at high cell density by chemostat culture based on pH-stat feeding. New ideas for obtaining magnetosomes can also be inspired by genetic engineering (Matsunaga et al. 2005). The adsorption capacity of MTB for Cd2+ has been improved by cell surface display technology (Tanaka et al. 2008); thus, more attention needs to be paid to this promising biosorbent. The application of MTB in removing and recovering heavy metals is emphasized in this paper. Ultrastructures and taxis of MTB are briefly introduced to promote the application of MTB.

Ultrastructures

Magnetosomes

Features

Magnetosomes consist of magnetite (Fe3O4) or greigite (Fe3S4) coated with membranes (Schüler and Frankel 1999). A combination of greigite and iron pyrite (FeS2), monoclinic pyrrhotite (Fe7S8), mackinawite (tetragonal FeS), and sphalerite-type cubic FeS were also identified in magnetotactic multicellular prokaryotes (MMP) (Bazylinski et al. 1995, 1993; Pósfai et al. 1998). These might be precursor phases of greigite crystals (Pósfai et al. 1998). Iron sulfide magnetosomes in MMP could contain variable concentrations of Cu (up to 10 at.%) (Bazylinski et al. 1993; Pósfai et al. 1998). Cu content may depend on Cu availability and is independent of the mineral phase (Pósfai et al. 1998). Only cobalt and manganese were successfully doped into magnetite particles (Keim et al. 2009; Staniland et al. 2008). Most magnetosomes are ferromagnetic with a size range of 35 nm to 120 nm (Bazylinski and Frankel 2004), whereas smaller particles in the superparamagnetic range (<30 nm) have also been observed (Fischer et al. 2008).

The morphologies and intracellular organization of magnetosomes in different MTB are shown in Fig. 1 (Schüler and Frankel 1999). Magnetosomes scattered throughout the cytoplasm or forming unordered clusters were also reported (Cox et al. 2002; Lin and Pan 2009). The scattering of magnetosomes is attributed to the different stages of magnetosome formation or different environmental conditions. The possibility of genetic mutation could not be ruled out as well. Another possibility is that the clustered magnetosomes are formed by a different biological mechanism and that they serve a different function to magnetosomes in chains.

Fig. 1
figure 1

Crystal morphologies and intracellular organization of magnetosomes in different MTB: a cubo-octahedral, b, d, e, and f elongated hexagonal prismatic, and c bullet-shaped. Magnetosomes are arranged in one (a, b, and c), two (e), or multiple chains (d) or irregularly (f). (Bar equivalent to 100 nm) Copyright © Springer-Verlag. Reproduced with permission

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The size and number of magnetosomes can vary considerably in response to culture medium composition and oxygen concentration (Cox et al. 2002; Heyen and Schüler 2003; Kundu et al. 2009; Liu et al. 2008). A correlation between pO2 and magnetosome formation was found in M. gryphiswaldense, Magnetospirillum magnetotacticum and Magnetospirillum sp. AMB-1. Magnetite biomineralization was induced only below a threshold value of 20 mbar O2 in all three strains (Heyen and Schüler 2003). High concentrations of Zn and Ni salts in the growth medium lead to the modification of bacterial and magnetosome properties, especially size, number, and chain alignment of M. magnetotacticum MS-1. The physical properties of MS-1 that are cultured in different media are shown in Table 1, which reveals that the formation of magnetosomes can be influenced and enhanced by chemical manipulation (Kundu et al. 2009).

Table 1 Physical properties of MS-1 cultured in different media (Copyright © Elsevier Ltd. Reproduced with permission)
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Flagella

The migration of MTB along geomagnetic field lines depends on the coordinated movement of flagella (Frankel 2003; Van Kampen 1995). Different amounts of flagella are found in various strains of MTB. Desulfovibrio magneticus RS-1 is propelled by a single polar flagellum at its tail end (Sakaguchi et al. 2002). M. gryphiswaldense cells are flagellated in its two poles (Schüler and Frankel 1999). Flagella that originate from a depression on the surface of some uncultured magnetotactic cocci are organized in bundles (Freitas et al. 2003). A mathematical model on the motion of assumptive spherical MTB with a single flagellum was put forward (Nogueira and Lins De Barros 1995).

Granules

Different sizes of granules without any membranes are found in most uncultured MTB. Phosphorus and oxygen are the most common elements in granules. Granules can be classified into two types: phosphorus-rich granules (P granules) and phosphorus–sulfur–iron granules (PSFe granules) (Keim and Farina 2005; Keim et al. 2005; Lins and Farina 1999; Zhu et al. 2010). In rod-shaped bacteria, these granules are positioned symmetrically around the center of the cell. P granules may naturally contain small amounts of manganese. Other elements, such as Na, Al, K, Ca, Zn, Cd, Fe, and Cl, are also detected on some occasions. PSFe granules have similar elemental composition (Keim and Farina 2005; Keim et al. 2001, 2005; Lins and Farina 1999).

Granules are considered as a major site for metal accumulation. Differences in the composition of granules may lead to differences in binding with metal ions. One compelling evidence is the accumulation of larger amounts of gold in PSFe granules than in P granules (Keim and Farina 2005). The capability of granules to assimilate metal ions may indicate a detoxification role. Meanwhile, high solubility of granules allows iron to be further used in metabolism (Byrne et al. 2010; Keim et al. 2005; Lins and Farina 1999). In addition, granules are also identified as lipid storage facilities and may act as storage compounds for energy and carbon needed in maintaining metabolism and synthesis of cellular metabolites during starvation. Another possible function of the large granules is buoyancy regulation (Cox et al. 2002; Keim et al. 2005; Zhu et al. 2010).

Capsules

Capsules are on the cell surface of uncultured MTB (Freitas et al. 2003). They are formed by numerous fibrils that extend to hundreds of nanometers from the cell surface. Capsules are maintained by the combination of electrostatic and hydrophobic–hydrophilic forces. They are known to mineralize different minerals, and most of them contain abundant iron (Freitas et al. 2003; Keim and Farina 2005).

Sulfur globules

Globules are intracellular structures in many strains of MTB (Cox et al. 2002; Zhu et al. 2010). MTB collected from the OATZ of a chemically stratified marine environment characterized by O2/H2S inverse double gradients form S globules (Bazylinski et al. 2004). S globules have no fixed number or specific location (Keim et al. 2005). They may act as electron donor reserves when cells travel from sulfide-rich to sulfide-depleted zones (Cox et al. 2002; Kopp and Kirschvink 2008).

S-layers

The surfaces of most uncultured magnetotactic cocci have S-layers, which are formed by particles organized in hexagonal symmetry (Freitas et al. 2003). S-layers are weakly acidic and completely cover the cell in all stages of growth in both bacteria and archaea. They are supposed to work as protective coats, molecular sieves, and as molecule and ion traps (Sleytr and Beveridge 1999). S-layers in MTB may relate to magnetotaxis. They may inhibit metal precipitation on the cell surface and influence the ability to overcome environmental viscous drags while moving to a suitable place (Freitas et al. 2003).

Membrane vesicles

The cell surfaces of different M. magnetotacticum cells and cocci have numerous spherical or oval-shaped membrane vesicles (Freitas et al. 2003; Lins et al. 2003). The function of vesicles and the chemical nature of these vesicular interiors have not been determined yet.

Taxis

Magnetotaxis

Axial MTB cells can swim in both directions along magnetic field lines, whereas the polar ones swim either parallel to geomagnetic field lines toward the north pole or in an unparallel direction toward the south pole (Frankel 2003; Lefevre et al. 2009). The magnetic polarity of MTB can be changed by applying an artificial magnetic field opposite to the geomagnetic field. Magnetotaxis enable MTB to swim between electron donor-rich and electron acceptor-rich regions (Frankel and Bazylinski 1994). Hence, magnetotaxis turn a three-dimensional problem into a one-dimensional one along the magnetic field, thereby providing a way to short-circuit the diffusion of MTB. The presence of magnetotaxis may also be an evolutionary exaptation; they serve as a metabolic pathway adapted for sensitivity to the geomagnetic field after the original evolution (Kopp and Kirschvink 2008).

Aerotaxis

When a magnetic field and an oxygen gradient are in the opposite direction, MTB hold a position in the OATZ instead of moving down magnetic field lines and accumulating at the area with the lowest oxygen concentration (Bazylinski and Frankel 2004). When a magnetic field toward the cuvette bottom is applied, the aerotaxis of AMB-1 cells occurs if the sample is exposed to atmospheric oxygen. The migration patterns could also be observed without a magnetic field (Smith et al. 2006).

Oxygen is a determinant factor that controls magnetotactic behavior (Lefevre et al. 2009). Modeling the magneto-aerotaxis of AMB-1 indicates that magnetotaxis may enhance the detectability for oxygen but not increase the average speed for moving away from high oxygen concentrations (Smith et al. 2006). Thus, magneto-aerotaxis help MTB move toward a preferred oxygen concentration more efficiently (Bazylinski and Frankel 2004; Frankel et al. 1997; Smith et al. 2006; Xie et al. 2009).

Phototaxis

The relationship between the phototaxis and magnetotaxis of AMB-1 was analyzed by Chen et al. (2011). AMB-1 migrates toward pure white light sources and multiple spectral bands; migration increases with a higher light intensity. The response to light is independent of wavelength and magnetotaxis. Magnetosome synthesis or magnetosome-containing cells increase after exposure to illumination for 20 h. Studies showed that marine magnetic spirilla cells respond to illumination (Chen et al. 2011; Zhu et al. 2010). However, nearly half of them change their swimming direction under the illumination of light with wavelengths from 330 nm to 550 nm, and the other half appear to lose motility and then become agglomerated.

Thermotaxis

When illuminated from the left and heated from the opposite side to create a temperature gradient of 0.1 °C, AMB-1 cells migrate toward both light and heat simultaneously, indicating that AMB-1 cells have a thermotactic response, which is independent from their phototactic response (Chen et al. 2011). Multiple taxis, which may result from a great number of chemoreceptor-like proteins, help MTB search for and move to a more suitable habitat to avoid certain disadvantages more efficiently (Chen et al. 2011; Frankel 2003; Frankel et al. 1997).

Application in removing heavy metals

Adsorption of heavy metals

Adsorption in unitary ion system

The removal of heavy metals and trace radionuclide from wastewater using MTB was first introduced by Bahaj et al. (1994, 1998a, 1991, 1998b). The removal efficiency of heavy metals by MTB was further studied (Song et al. 2008; Wang et al. 2011), as shown in Table 2. The maximum amount of Cd2+ that could bind to 1.0 × 1010 wild-type AMB-1 cells was 60 nmol or more. High accumulation of arsenic and tellurium (Te) ions on the cell surface of AMB-1 cells was also verified. In obtaining better effects and convenience, studies on bioremediation are usually based on the entire MTB group instead of only on a certain strain of MTB. The removal efficiency of Cu (II) in the pH range of 2.0 to 4.5 was 98.07 % to 98.75 %, much higher than that of M. gryphiswaldense MSR-1. The adsorption yield of Au (III) on MTB was 99.53 % to 100 % in the pH range of 1.0 to 5.5 (Song et al. 2007; Wang et al. 2011). Both Langmuir and Freundlich adsorption models were suitable for describing the biosorption equilibrium of Cu (II) ions or Au (III) on MTB, but the Langmuir model matched better with adsorption of Au (III). The maximum adsorption capacity of some biosorbents for Au (III) is summarized in Table 3 (Song et al. 2008, 2007). Furthermore, a genetic modification technique could improve the adsorption ability of MTB. A hexahistidine amino acid was inserted to the membrane-specific protein of AMB-1 and then displayed on the cell surface using cell surface display technology. The quantity of Cd2+ absorbed by the AMB-1 transformant was 40 % higher than that of the wild-type AMB-1, whereas the collection efficiencies of both types of bacteria recovered by magnetic force were almost the same (Tanaka et al. 2008). Details on biosorption are still insufficient because of the limited amount of data, but MTB are obviously viable biosorbents.

Table 2 Removal efficiency of heavy metals by MTB (25 °C) (Song et al. 2008; Wang et al. 2011)
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Table 3 Maximum adsorption capacity of biosorbents for Au (III) (Song et al. 2007, 2008)
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The mineralization of metal ions occurs in MTB after exposure to metallic elements other than iron (Arakaki et al. 2002; Cai et al. 2011; Song et al. 2008; Tanaka et al. 2010). Monodisperse nanocrystals composed of Cd, S, and P were observed on the RS-1 cell surface after growing in the presence of Cd2+. More than 95 % of cadmium at an initial concentration of 1.3 mg L−1 was removed by precipitation after cultivation for 240 h. Sulfate-reducing bacteria could precipitate a variety of elements by reduction, including As (V), U (VI), and Tc (VII). Therefore, RS-1 may also accumulate metallic elements other than Cd (Arakaki et al. 2002). After 24 h of exposure to metals, large amounts of gold were trapped by the PSFe granules of magnetotactic cocci, and some were found on the capsules. Crystalline silver sulfide deposits were in the cell envelope (Keim and Farina 2005). The biosynthesis of gold nanoparticles was further studied using MSR-1. Spherical gold nanoparticles accumulated on the cell surfaces through 1 h of biosorption. In the pH range of 1.5 to 4.0, the higher the pH value, the larger the diameter of the gold nanoparticles. Moreover, particle diameter increased with the initial metal concentration. Thus, the size of gold particles can be controlled by the pH value and the initial metal concentration (Cai et al. 2011). Rod-shaped Te nanoparticles were distributed along the magnetosome chain in AMB-1. The number and size of Te crystals increased with the initial Te concentration to the medium, but the formation of magnetite crystals decreased gradually (Tanaka et al. 2010).

Most metallic elements stably existed on the cell surface by binding to the membrane or in the form of inorganic precipitates (Arakaki et al. 2002; Keim and Farina 2005; Keim et al. 2005). Functional proteins on the cell surface, capsules, and granules may play important roles in biomineralization (Cai et al. 2011). Differences in the composition of ultrastructures may lead to the binding of different metal ions. For instance, larger amounts of gold were trapped by a PSFe granule-type of uncultured MTB, whereas silver-containing deposits were in a P granule-type of uncultured MTB (Keim and Farina 2005).

The removal of Te by cell surface adsorption was two orders of magnitude less than the accumulation achieved by crystallization, indicating that chemisorption and biomineralization are important ways of removing heavy metals (Tanaka et al. 2010). Moreover, the minimum inhibitory concentration of heavy metals for MTB is relatively high (Kundu et al. 2009). Biosorption and biomineralization can be controlled by regulating the adsorption conditions (exposure time to metals, pH value, initial metal concentration, and others). Therefore, MTB not only serve as ideal biosorbents, they also provide an organic or natural method for synthesizing metallic nanoparticles.

Adsorption in binary ion system

A variety of metal elements are contained in industrial wastewater. The adsorption of MTB in binary ion systems has been studied. When the initial concentrations of Au (III) and Cu (II) were below 80 mg L−1, the final concentration of both ions in wastewater were less than 1.0 mg L−1 after MTB adsorption. The adsorption of Au (III) was reinforced (95.87 %) and that of Cu (II) was prohibited (8.83 %) with a coexisting concentration of 320 mg L−1 (Song et al. 2007). For the biosorption of MSR-1 in an Ag–Cu system, adsorption for both metallic elements was promoted when the molar ratio of Ag (I) and Cu (II) was below 4:1. Adsorption for Cu (II) decreased, whereas that for Ag (I) was enhanced when the ratio was over 4:1 (Wang et al. 2011). Selective reinforced competitive biosorptions of MTB in binary ion systems occur due to the functional groups and proteins on cell surfaces as well as physical and chemical properties of heavy metals (Song et al. 2007; Wang et al. 2011).

Adsorption conditions

Adsorption conditions include temperature, pH value, biomass concentration, stirring, and pretreatments. The removal efficiencies were almost the same in the range of 10 °C to 35 °C. One possible reason is that the adsorption of metal ions on MTB was mainly chemisorption and ion exchange instead of physical adsorption, which could be easily influenced by temperature (Song et al. 2007; Wang et al. 2011). A higher adsorption effect for heavy metals was seen in highly acidic conditions. The adsorption amounts of Cu (II) and Au (III) increased slightly within the pH range of 1.0 to 4.0 (Wang et al. 2011). As pH increased, more ligands with negative charges on the cell wall would be exposed, and the attraction to positively charged metal ions would increase. In the study of Song et al., however, the adsorption of Cu (II) in the pH range of 2.0 to 4.5 showed no significant difference, as well as that of Au (III) in the pH range of 1.0 to 5.5 (Song et al. 2007).

The adsorption amount rose with biomass concentration within limits. Usually, an appropriate stirring would accelerate adsorption. The removal efficiency increased rapidly in the first 30 min when samples were agitated on a shaker at 130 rpm and adsorption equilibrium was reached in 1 h (Song et al. 2008, 2007; Wang et al. 2011).

Pretreatments, such as drying and treating with sodium hydroxide and hydrochloric acid, would lower adsorption efficiency. This result may be because lipids and proteins in the cell wall involved in biosorption were damaged, thereby reducing the number of binding sites (Das 2010; Keim et al. 2001; Song et al. 2008; Wang et al. 2011).

Separation and desorption of MTB

A low magnetic field separation system for metal-loaded MTB was introduced by Bahaj et al (1998b). A magnetic separator was developed (Fig. 2), by which MTB can be easily separated from aqueous solutions (Song et al. 2008). A pair of NdFeB magnets was placed on two sides of the separator to generate a permanent magnetic field that was vertical to the flowing direction of the feed. The intensity of the magnetic field could be adjusted by changing the distance between the magnets. A high-gradient magnetic field was generated by fixing multilayers of nickel wires inside the separator. Nickel wires were parallel to the flow direction and worked as a trapping matrix to capture MTB. Thus, metal-loaded MTB could be separated and recovered from wastewater. MTB loaded with Au (III) were quickly removed in the first 30 min. The entire biomass could be removed and accumulated on the nickel wires at the magnetic intensity of 1,200 Gs in 180 min. Desorbents (e.g., HNO3, EDTA, NaNO3, and thiourea) were used to elute the Au adsorbed on MTB, among which thiourea proved to be an effective desorbent. About 91.0 % of Au could be recovered from the MTB biomass with the thiourea concentration of 0.8 M. Hence, applications of MTB in the removal and recovery of heavy metals have a promising future.

Fig. 2
figure 2

Schematic diagram of magnetic separator. 1 Inlet, 2 liquid distributor, 3 fixed frame, 4 outlet, 5 Plexiglass container, 6 nickel wires. Copyright © Elsevier Ltd. Reproduced with permission

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Prospects

The application of MTB in removing heavy metals has a notable potential. Biosorption and biomineralization are two efficient ways of removing heavy metals. These methods can be controlled by regulating the adsorption conditions. With the help of genetic modification technique, the adsorption ability of MTB will be further enhanced. Moreover, metal-loaded MTB can be easily separated from wastewater by a magnetic separator. The present paper contributes toward an energy-efficient way of recovering precious metals by adsorption and producing metal nanoparticles. More research into selective reinforced competitive biosorption of MTB for heavy metals is needed.

More details on ultrastructures in MTB are required to shed light on the adsorption and reduction law of MTB on heavy metals. In-depth knowledge about the taxis of MTB will also be useful when studying bulk culture samples and their applications. The application of MTB in removing and recovering heavy metals will be further promoted on the basis of a thorough understanding of MTB.